This patch adds texinfo-4.13a back to Guix, as an alternative to
texinfo-5.2.

The motivation is that building Guile's manual using texinfo-4.13a takes
about 6 seconds on my Yeeloong, compared with about 12 *minutes* using
texinfo-5.2.  For me, that's intolerable.  I think that anyone with a
slower machine who wants to get work done on a project with a large
texinfo manual will want this.

What do you think?

      Mark